EP0977075B1 - Verfahren zur Herstellung eines optischen Verzögerungsfilms - Google Patents

Verfahren zur Herstellung eines optischen Verzögerungsfilms Download PDF

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Publication number
EP0977075B1
EP0977075B1 EP99114401A EP99114401A EP0977075B1 EP 0977075 B1 EP0977075 B1 EP 0977075B1 EP 99114401 A EP99114401 A EP 99114401A EP 99114401 A EP99114401 A EP 99114401A EP 0977075 B1 EP0977075 B1 EP 0977075B1
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EP
European Patent Office
Prior art keywords
substrate
rollers
rubbing
angle
roller
Prior art date
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EP99114401A
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English (en)
French (fr)
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EP0977075A1 (de
Inventor
Keith Hanrahan
John Scott
Mark Dr. Goulding
Mark Andrew Verrall
David Dr. Coates
Nicolas Sharples
Jun Nakanowatatri
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Merck Patent GmbH
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Merck Patent GmbH
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    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G02F1/13363—Birefringent elements, e.g. for optical compensation
    • C—CHEMISTRY; METALLURGY
    • C09—DYES; PAINTS; POLISHES; NATURAL RESINS; ADHESIVES; COMPOSITIONS NOT OTHERWISE PROVIDED FOR; APPLICATIONS OF MATERIALS NOT OTHERWISE PROVIDED FOR
    • C09K—MATERIALS FOR MISCELLANEOUS APPLICATIONS, NOT PROVIDED FOR ELSEWHERE
    • C09K19/00—Liquid crystal materials
    • C09K19/04—Liquid crystal materials characterised by the chemical structure of the liquid crystal components, e.g. by a specific unit
    • C09K19/38—Polymers
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F1/00—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics
    • G02F1/01—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour 
    • G02F1/13—Devices or arrangements for the control of the intensity, colour, phase, polarisation or direction of light arriving from an independent light source, e.g. switching, gating or modulating; Non-linear optics for the control of the intensity, phase, polarisation or colour  based on liquid crystals, e.g. single liquid crystal display cells
    • G02F1/133—Constructional arrangements; Operation of liquid crystal cells; Circuit arrangements
    • G02F1/1333—Constructional arrangements; Manufacturing methods
    • G02F1/1335—Structural association of cells with optical devices, e.g. polarisers or reflectors
    • G02F1/13363—Birefringent elements, e.g. for optical compensation
    • G02F1/133633—Birefringent elements, e.g. for optical compensation using mesogenic materials
    • G—PHYSICS
    • G02—OPTICS
    • G02F—OPTICAL DEVICES OR ARRANGEMENTS FOR THE CONTROL OF LIGHT BY MODIFICATION OF THE OPTICAL PROPERTIES OF THE MEDIA OF THE ELEMENTS INVOLVED THEREIN; NON-LINEAR OPTICS; FREQUENCY-CHANGING OF LIGHT; OPTICAL LOGIC ELEMENTS; OPTICAL ANALOGUE/DIGITAL CONVERTERS
    • G02F2413/00—Indexing scheme related to G02F1/13363, i.e. to birefringent elements, e.g. for optical compensation, characterised by the number, position, orientation or value of the compensation plates
    • G02F2413/10—Indexing scheme related to G02F1/13363, i.e. to birefringent elements, e.g. for optical compensation, characterised by the number, position, orientation or value of the compensation plates with refractive index ellipsoid inclined, or tilted, relative to the LC-layer surface O plate

Definitions

  • the invention relates to a process for preparing an optical retardation film according to claim 1.
  • the invention further relates to a method of rubbing a substrate for the alignment of liquid crystal or mesogenic material that can be used in the above process.
  • Optical retardation films comprising anisotropic polymer material with uniform orientation - like for example an oriented liquid crystal polymer - can be used as compensation films for liquid crystal displays, in order to compensate a deterioration of the optical properties of the display at large viewing angles, such as a colour shift or a decrease of the contrast ratio. They can also be used for example as quarter wave retardation films (QWF) in order to transform circularly polarized light into linear or elliptically polarized light or vice versa.
  • QWF quarter wave retardation films
  • Uniform orientation of a liquid crystal material is typically achieved by coating the material onto a substrate that is covered by an alignment layer, the surface of which may additionally have been rubbed in one direction.
  • the surface material and the rubbing of the surface induce spontaneous uniform orientation of the liquid crystal material.
  • various orientations can be achieved depending on the type of material of the alignment layer various orientations can be achieved.
  • the prior art also provides various methods for rubbing a substrate.
  • rubbing can be carried out by means of a rubbing roller.
  • the International Application WO 98/04651 -A discloses an optical retardation film that can be used as a QWF in a display and comprises a layer of polymerized mesogenic material with uniform planar orientation, i.e. with its major optical axis being oriented substantially parallel to the film plane.
  • the WO 98/04651-A further discloses a method of preparing such an optical retardation film, according to which a polymerizable mesogenic material is coated onto a substrate that has been rubbed directly and that causes planar alignment of the mesogenic material, followed by curing the polymerizable material e.g. by exposure to heat or UV radiation.
  • the WO 98/12584-A discloses an optical compensation film for a display and a process of its preparation, wherein the compensation film comprises a layer of polymerized mesogenic material with splayed orientation, i.e. with its major optical axis being tilted at an angle to the normal of the film and the tilt angle varying in vertical direction to the film plane.
  • the process of preparing the compensation film described in the WO 98/12584-A also implies the use of a substrate the surface of which has been rubbed directly to induce or improve alignment of the mesogenic material.
  • a long roll of a retardation film with planar orientation as described in the WO 98/04651-A can be prepared.
  • the retardation film is then cut at an angle into small sheets and laminated to the linear polarizer, with the cutting angle being identical to the above mentioned inclination angle of the optical axes of polarizer and retardation film.
  • a less time- and material-consuming method would be the preparation of a long roll of a retardation film having planar orientation, wherein the optical axis already exhibits the desired angle of inclination relative to the film length.
  • This retardation film could then be directly laminated roll to roll to the linear polarizer, and afterwards the laminated films could be cut into sheets of the desired size.
  • the method should preferably comprise the steps of rubbing a long film substrate and preparing the optical film on the rubbed substrate in a single, continuous process.
  • optical axis or “major optical axis” are used to describe the optical symmetry of mesogenic or liquid crystalline materials and optical retardation films comprising them.
  • the major optical axis is given by the average preferred orientation direction of the long axes of the liquid crystal molecules, whereas in case of an optical unaxially negative nematic liquid crystal material the major optical axis is given by the orientation direction of one of the short axes of the liquid crystal molecules.
  • a retardation film with planar orientation and an optical axis inclined to the film length and width can be prepared for example by coating a liquid crystal material onto a film substrate that has been rubbed at an angle different from 0 degrees to its long side.
  • This method is also commonly called off-axis rubbing and is well-known in prior art. Thereby, it is possible to achieve alignment of the liquid crystal molecules with their long molecular axes inclined at an angle to the long side of the film, wherein said angle of inclination of the liquid crystal molecules and the angle at which the substrate has been rubbed are approximately the same.
  • a typical method of off-axis rubbing is carried out by moving a substrate across a rotating rubbing roller, wherein the rotation axis of the rubbing roller is inclined at an angle, the so-called rub angle, to the moving direction of the substrate.
  • rub angle By varying the rub angle, it is possible to alter the orientation of the liquid crystal material coated onto said substrate.
  • US 5,853,801 discloses a process for preparing a compensator by coating a layer of discotic liquid crystal on a rubbed substrate, but does not disclose off-axis rubbing.
  • One of the aims of the present invention is to provide a method of off-axis rubbing of a substrate which can be used for the above mentioned purposes. Another aim is to provide a process of preparing an optical retardation film fulfilling the above mentioned requirements. Other aims of the present invention are immediately evident to the skilled in the art from the following detailed description.
  • One object of the present invention is a process for preparing an optical retardation film comprising at least one layer of an anisotropic polymer material, according to claim 1.
  • Another object of the present invention relates to a method of rubbing a substrate for aligning liquid crystal material coated onto said substrate.
  • Another object of the present invention is an apparatus that is used for the inventive method of rubbing.
  • Figure 1 exemplarily illustrates a method of rubbing a substrate with a rubbing roller according to a first preferred embodiment of the present invention, with Figure 1A showing the plan view and Figure 1B showing the cross section for a rub angle of 0 degrees.
  • rubbing is carried out by continuously moving a substrate 10 into the direction 13;13* between at least two rollers 11 and 12, at least one of which is giving rubbing treatment to the substrate, wherein
  • the term 'continuously moving' means that the substrate is constantly moving, i.e. without being stopped, into one direction during the rubbing process, preferably with constant speed.
  • the substrate can for example be drawn through the rubbing and compensation roller 11;12 by a wind up roller that is located downstream of the rollers 11;12 and is operated by a constant speed motor (not shown in Figure 1).
  • the moving speed and tension of the substrate can thus be controlled by the rotation speed of the wind up roller.
  • the lateral position of the wind up roller consequently has to be adjusted to allow for the change of the substrate position (as indicated by the different lateral positions of arrows 13 and 13* in Figure 1A).
  • the wind up roller is not exactly in line with the roll from which the substrate is being unwound.
  • further rollers can be used to control the moving direction, speed and tension of the substrate.
  • the moving substrate is preferably held in its moving direction 13;13* by further guide rollers 16 and 17.
  • At least one one of the rubbing rollers is contributing to the rubbing required for aligning the liquid crystal material, whereas at least one other of the rubbing rollers is a counter roller which acts to prevent the substrate from drifting.
  • rubbing roller denotes a roller that is located on the side of the substrate which subsequently is coated with the liquid crystal material
  • the term “compensation roller” is used for a counter roller located on the opposite side of the substrate.
  • roller 11 is being referred to as the "rubbing roller”
  • roller 12 is being referred to as the "compensation roller”.
  • the two rollers 11 and 12 are also interchangeable with each other with respect to their function, e.g. by inverting the moving direction of the substrate or by using the opposite side of the substrate for the coating with LC material.
  • the inventive method of rubbing is characterized in that the rub angle, which is defined as the angle between the rotation axis of the rubbing roller 11 and the normal to the direction of the moving substrate 13;13*, can be varied from 0 to less than 90 degrees. This can be easily achieved for example by turning the rubbing roller 11 around the centre of its rotation axis 14 in a plane parallel to the plane of the substrate 10.
  • the inventive method is further characterized in that the substrate is wrapped at least partially around the rubbing roller and preferably also around the compensation roller.
  • wrapped at least partially around a roller herein means that the substrate is moved over the roller in such a way that a definite surface area of the substrate is in contact with the roller.
  • wrap angle ⁇ is the angle formed by the two radii at the tangential positions 18 and 19 between which the rubbing roller 11 is in contact with the substrate 10.
  • a wrap angle of 0 degrees means that the substrate 10 is moved across the rubbing roller 11 without changing its direction, whereas for example a wrap angle of 180 degrees means that the substrate performs a U-turn when being moved across the rubbing roller 11.
  • the wrap angle ⁇ and thereby the rub length can easily be varied for example by a translational shift of the rotation axis 14 of the rubbing roller 11 and/or the rotation axis 15 of the compensation roller 12 in vertical 20 and/or horizontal 21 direction as depicted by the respective arrows.
  • Figure 2 shows a rubbing method according to a second preferred embodiment of the present invention, wherein the rotation axes 14;15 of the two rollers 11;12 are connected by at least one linking element 22 to form a unit.
  • the wrap angle ⁇ and thereby the rub length can easily be varied by turning the connected rollers 11 and 12 around an axis 23 parallel to their rotation axes 14;15.
  • the diameters the rollers 11 and 12 are identical, and the axis 23 is in the center between the two rotation axes 14 and 15, as depicted in Figure 2.
  • the axis 23 is in the plane 24 of the tangential lines at which the substrate meets the guide roller 16 and at which the substrate leaves the guide roller 17.
  • Figure 3 shows an apparatus for rubbing a substrate according to a third preferred embodiment of the present invention.
  • the rubbing roller 11 and the compensation roller 12 form a rotatable unit 25 that further comprises a driving means 26 comprising one or more motors and optionally gears giving rotation speed of equal absolute value and opposite sense to the rollers 11;12.
  • unit 25 is rotatable around two axes 27 and 28.
  • the rub angle ⁇ can easily be varied by rotating the unit 25 clockwise or counterclockwise into the directions shown by the arrow 29 around the axis 27 that is perpendicular to the plane defined by rollers 16 and 17, whereas the wrap angle ⁇ can be varied by rotating the unit around the axis 28 that is parallel to the rotation axes 14;15 of the rollers 11;12.
  • Figure 4 shows an apparatus for rubbing a substrate according to a fourth preferred embodiment of the present invention, which comprises a unit 25 with a rubbing roller 11 and a compensation roller 12, a driving means 26 comprising one or more motors and optionally gears for rotating the rollers, and further comprises two vacuum beds 30 and 31 over which the substrate is moved before and after being rubbed.
  • the vacuum beds act as a support to the moving substrate 10 that stabilizes the substrate in its moving direction 13;13* and in addition to the compensation roller suppresses or, in the ideal case, avoids a possible side shift of the substrate 10 from its moving direction 13;13* that may eventually be caused for example by a difference in friction of the moving rollers 11;12 on the substrate, e.g. due to a speed difference between the rollers.
  • Vacuum beds are used in prior art e.g. to hold a film or sheet substrate in a fixed place or fixed moving direction and are known to the expert in the field.
  • they consist of a metal box having a top plate with a plurality of holes onto which the substrate is placed, or across which the substrate is moved, and a means to reduce the pressure inside the box. (This reduction of pressure is also commonly called “application of a vacuum”).
  • vacuum beds are not necessarily required in all cases and under all conditions.
  • only one vacuum bed is used, over which the substrate is moved either before being rubbed (bed 30) or after having been rubbed (bed 31).
  • the rubbing roller 11 and optionally also the compensation roller 12 are preferably covered with a rubbing cloth. As mentioned above, however, usually only the rubbing roller contributes to the rubbing required, whereas the compensation roller is a counter roller which mainly acts to prevent the substrate from drifting.
  • both the rubbing roller 11 and the compensation roller 12 are covered with a rubbing cloth.
  • the compensation roller is also rubbing the substrate on the opposite side, which in turn ensures that the friction is equal on both sides of the substrate.
  • both sides of the substrate are rubbed, either side of the substrate can be used for the alignment of a liquid crystal material being coated thereon.
  • both rollers 11 and 12 are covered with a rubbing cloth, both sides of the substrate 10 can be coated with liquid crystal material, e.g. subsequently one after another.
  • the guide roller 17 is preferably omitted, in order not to negatively affect the alignment properties of the surface of the substrate that was rubbed by the roller 12.
  • the rubbing cloth in addition to its function of rubbing the substrate, the rubbing cloth also reduces the friction of the substrate wrapped around the roller.
  • the rubbing cloth is preferably selected such that in the ideal case it gives point contact to the substrate, with the substrate floating on the points.
  • point contact in this connection has the following meaning:
  • a velvet cloth used as rubbing cloth this has a pile, i.e. it has many strands of cotton with uniform length that are attached to a base.
  • the film substrate in the inventive rubbing method comes in contact with these strands, e.g. if the distance from the base of the velvet to the film substrate is reduced to a value less than the length of the pile, each individual strand effects the film in the same way.
  • the ideal structure in this case would be that all strands slightly touch the film substrate, thus giving point contact to the substrate. However, in practice this normally does not occur due the pressure put on the base to keep it taut.
  • the rollers 11;12 are started to rotate before the substrate 10 is started to move. Thereby, an air cushion is produced, and the substrate is caused to float on top of the rubbing cloth, which reduces especially the initial friction of the substrate when contacting the rollers.
  • air is pumped from inside of the rollers 11;12 through small holes in the mantle surface of the rollers, in order to aid the formation of an air cushion as described above during the start of the rubbing process.
  • the air pressure is typically reduced to aid control of the floating of the substrate and thereby to aid control of the rubbing friction.
  • the above mentioned method to form an air cushion is particularly useful when film substrates are used that are sensitive to stress and/or friction, and especially when large wrap angles are applied.
  • rubbing cloth all conventional materials can be used.
  • velvet of a commercially available standard type can be used as a rubbing cloth.
  • the alignment properties of the substrate for a mesogenic or liquid crystal material coated on the rubbed substrate can be controlled by adjusting the process parameters of the inventive rubbing process, such as the rub angle ⁇ , wrap angle ⁇ , rub length, rubbing pressure, rubbing depth, the rotation velocity and diameter of the rubbing roller and compensation roller and the tension on the substrate.
  • v l denotes the direction and constant speed of a moving substrate
  • R denotes the axis of a rubbing roller located at the rub angle ⁇ relative to the direction p perpendicular to the moving direction of the substrate in the plane of the substrate, wherein said rubbing roller has the diameter d (not shown) and rotates at the constant v rol given in rounds per time (not shown).
  • the angle ⁇ eff in the following is also called the effective rub angle.
  • the rub length I is directly proportional to the wrap angle ⁇ , the diameter d of the rubbing roller and the rotation speed of the rubbing roller, and inversely proportional to the forward speed of the substrate.
  • the rub length can be calculated from the total rub speed v t and the wrap angle ⁇ as follows:
  • the preferred ranges of values for the process parameters such as the rub angle, wrap angle, rubbing pressure, rubbing depth, speed and size of the rubbing and compensation rollers, and speed of the moving web can be easily selected and adjusted by the expert to achieve the desired effective rub angle and rub length, and thereby to control the orientation of the liquid crystal molecules coated onto the substrate.
  • the rub angle ⁇ in the inventive method is preferably from 0 to 75 degrees, in particular from 1 to 60 degrees, very preferably from 3 to 45 degrees.
  • the wrap angle ⁇ in the inventive rubbing method is preferably from 0 to 270, especially from 0 to 180, very preferably from 0 to 120 degrees.
  • the wrap angle ⁇ in the inventive rubbing method can be varied from 0 up to almost 270 degrees if the diameter of the rubbing roller 11 is equal to or smaller than the diameter of the compensation roller 12, as obvious from Figures 1 and 2.
  • Figure 6 depicts another preferred embodiment of the present invention.
  • the wrap angle ⁇ can be increased beyond values of 270 degrees up to almost 360 degrees, if two compensation rollers 12 and 32 are used whose diameters are smaller than that of the rubbing roller 11.
  • further guide rollers 33;34 can be used in addition to roller 16.
  • both compensation rollers 12 and 32 like the diameter, rotation speed, rubbing pressure, surface material etc., have to be adjusted so that in total they compensate the respective parameters of the rubbing roller 11, in order to prevent a side drift of the substrate.
  • the wrap angle and thus the rub length can further be increased by using further rollers that rub the substrate in addition to rollers 11, 12 and 32.
  • one further object of the present invention is a rubbing apparatus comprising at least two rollers 11 and 12, at least one of which is giving rubbing treatment to a substrate 10 that is moved between said at least two rollers 11;12, and optionally comprising one or more guide rollers 16;17, wherein the position of said rollers and the conditions of said rubbing treatment are as described for the first preferred embodiment above.
  • Another object of the present invention is an apparatus as described above, wherein the rotation axes 14;15 of said at least two rollers 11;12 are connected by at least one linking element 22, and wherein the rollers 11;12 are rotatable around an axis 23 parallel to their rotation axes 14;15.
  • rollers 11 and 12 form a rotatable unit 25 that further comprises a driving means 26 comprising one or more motors and optionally gears giving rotation speed of equal absolute value and opposite sense to the rollers 11;12, and wherein unit 25 is rotatable around the two axes 27 and 28 as described for the fourth preferred embodiment above.
  • Another object of the present invention is an apparatus as described above that is further comprising one or more vacuum beds 30;31 over which the substrate 10 is moved before and/or after being rubbed.
  • Another object of the present invention is an apparatus as described above that is comprising at least three rollers 11;12;32 giving rubbing treatment to the substrate 10, at least two of which are located on opposite sides of the substrate 10.
  • the substrate is a flexible plastic film, such as for example a film of polyethyleneterephthalate (PET), polyvinylalcohol (PVA), polycarbonate (PC) or triacetylcellulose (TAC), especially preferably a PET film or a TAC film.
  • PET films are commercially available for example from ICI Corp. under the trade name Melinex. The above given examples of film materials should, however, only explain but not restrict the present invention.
  • Isotropic or birefringent substrates can be used.
  • the substrate is not removed from the coated film, preferably isotropic substrates are used.
  • a birefringent substrate for example an uniaxially stretched plastic film can be used.
  • the liquid crystal material can be coated onto the rubbed substrate after the rubbing process in a single, continuous process, e.g. on the same production line. It is further possible to laminate the optical retardation film prepared by the inventive method directly (e.g. roll to roll) to other films or layers, such as further optical film components or adhesions, protection or release films and the like.
  • the act of rubbing of the substrate and the act of coating of the liquid crystal material onto the substrate can be carried out separately, and the rubbed substrate can inbetween e.g. be stored and/or shipped.
  • the liquid crystal material that is used in the inventive process of preparing an optical retardation film can be selected principally from all types of liquid crystal materials known to the expert. Especially preferred materials are polymerizable mesogenic or liquid crystalline compounds or mixtures thereof, linear or crosslinked liquid crystal polymers of the side chain and/or main chain type, liquid crystal oligomers, or mixtures of these materials.
  • the liquid crystal material is coated on the moving substrate in form of a thin layer. This can be done by conventional techniques that are known to the expert, such as stream coating, doctor blading etc..
  • organic solvents such as ketones like e.g. methyl ethyl ketone or cyclohexanone, aromatic solvents like e.g. toluene or xylene or alcohols such as e.g. methanol, ethanol or isopropyl alcohol. It is also possible to use binary, ternary or higher mixtures of the above solvents.
  • the coated liquid crystal material usually aligns itself spontaneously to give the desired orientation, depending on the type of substrate and the rubbing direction.
  • An orientation of the liquid crystal material wherein the molecules orient themselves with their long axes parallel to the moving direction of the substrate can be achieved e.g. by coating the material onto a plastic substrate that had been rubbed in a direction parallel to the moving direction of the substrate.
  • An orientation of the liquid crystal material wherein the molecules orient themselves with their long axes inclined at an angle to the moving direction of the substrate can be achieved by off-axis rubbing of the substrate, as exemplarily shown in Figure 7.
  • Figure 7 schematically depicts a piece of an optical retardation film 35 that has been prepared according to a preferred embodiment of the present invention, by coating a liquid crystal material on a moving long film substrate which was previously subjected to off-axis rubbing at the rub angle ⁇ .
  • the x-y-plane represents the contact plane of film and substrate, and the x-axis represents both the long side and the moving direction of the substrate.
  • off-axis rubbing of the substrate induces an alignment in the liquid crystal material wherein the liquid crystal molecules are oriented parallel to the plane of the substrate, with the average preferred orientation direction 36 of the liquid crystal molecules being inclined at an angle ⁇ LC to the long side of the substrate.
  • the major optical axis of the resulting retardation film is also inclined at an angle ⁇ LC to the long side of the film, wherein the value of ⁇ LC is approximately the value of the effective rub angle ⁇ eff , which can be calculated from the rub angle ⁇ as described above.
  • the anchoring forces of the substrate surface are oriented such that the liquid crystal material is not oriented parallel, but tilted at an angle to the surface of the substrate. This situation is exemplarily shown in Figure 8.
  • Figure 8 schematically depicts a piece of an optical retardation film 37 that has been prepared according to another preferred embodiment of the present invention, by coating a liquid crystal material on a moving substrate which was previously subjected to off-axis rubbing at a rub angle ⁇ .
  • the x-y-plane represents the contact plane of film and substrate
  • the x-axis represents the long side and the moving direction of the substrate
  • the z-axis represents the direction normal to the film plane.
  • the average preferred orientation direction 38 of the liquid crystal molecules in addition to being inclined at an angle ⁇ LC to the long side of the film due to off-axis rubbing as described for Figure 7 above, is also tilted at an angle ⁇ o out of the film plane.
  • the tilt angle ⁇ o can be altered by varying the rubbing conditions, in particular the rub length, rubbing pressure, substrate material, roller surface material and the LC material. For example, by adding a surfactant to the liquid crystal material, it is possible to reduce the tilt angle ⁇ o and thereby to obtain planar alignment with almost zero tilt.
  • the substrate is coated with a layer of an organic material, preferably with polyimide, nylon or PVA, in order to change the alignment.
  • an organic material preferably with polyimide, nylon or PVA
  • An alignment layer can also be applied in order to increase the uniformity of alignment.
  • Uniform alignment of the coated liquid crystal material can further be enhanced for example by shearing the material. In some cases the shear forces caused by the act of coating itself are sufficient to improve the alignment.
  • Suitable techniques for further treatment of the substrate surface can be found in the literature, e.g. in the publications of J. Cognard, Mol.Cryst.Liq.Cryst. 78, Supplement 1, 1-77 (1981) and J.A. Castellano, Mol.Cryst.Liq.Cryst. 94, 33-41 (1983 ).
  • the inventive process of preparing an optical retardation film is carried out by using a substrate the surface of which has been rubbed directly, without the application of an alignment layer on top of the substrate.
  • the coated layer of the liquid crystal material is covered with a second substrate, wherein the shearing caused by putting together the two substrates improves the alignment.
  • nematic liquid crystal materials especially nematic liquid crystalline mixtures of polymerizable mesogenic compounds, however, these materials tend to align easily on a rubbed surface so that further treatment is usually not necessary.
  • the optical retardation film is prepared from a polymerizable mesogenic material which is coated on the rubbed substrate, where it is aligned into a uniform orientation, and subsequently cured by exposure to heat or actinic radiation in the presence of a polymerization initiator.
  • the substrate can be removed after polymerization or not. In case two substrates are used, at least one of the substrates has to be transmissive for the actinic radiation used for the polymerization.
  • the polymerizable mesogenic material is preferably comprising at least one polymerizable mesogenic compound having at least one polymerizable group.
  • polymerizable mesogenic compound or polymerizable liquid crystalline compound as used in the foregoing and the following comprise compounds with a rod-shaped, board-shaped or disk-shaped mesogenic group, i.e. a group with the ability to induce mesophase behaviour in a compound comprising said group. These compounds do not necessarily have to exhibit mesophase behaviour by themselves. It is also possible that these compounds show mesophase behaviour only in mixtures with other compounds or when the polymerizable mesogenic compounds or the mixtures comprising them are polymerized.
  • a preferred embodiment of the present invention relates to a process of preparing an optical retardation film, wherein the polymerizable mesogenic material comprises
  • Suitable polymerizable mesogenic compounds and mixtures that are preferably used in the inventive process can be found in the WO 98/04651-A , WO 98/00475 and WO 98/12584-A .
  • x and y are each independently 1 to 12
  • F is a 1,4-phenylene or 1,4-cyclohexylene group
  • R 0 is halogen, cyano or an optionally halogenated alkyl or alkoxy group with 1 to 12 C atoms
  • L 1 and L 2 are each independently H, F, Cl, CN, or an optionally halogenated alkyl, alkoxy or alkanoyl group with 1 to 7 C atoms.
  • the polymerizable mesogenic compounds of V1-V7 can be prepared by methods which are known per se and which are described in standard works of organic chemistry such as, for example, Houben-Weyl, Methoden der organischen Chemie, Thieme-Verlag, Stuttgart. Further methods of their preparation can be taken for example from the above cited WO 93/22397 ; EP 0 261 712 or DE 195,04,224 .
  • polymerizable mixtures comprising at least two reactive mesogenic compounds at least one of which is a compound of formula V1-V7.
  • Polymerization of the polymerizable mesogenic material takes place by exposing it to heat or actinic radiation.
  • Actinic radiation means irradiation with light, like UV light, IR light or visible light, irradiation with X-rays or gamma rays or irradiation with high energy particles, such as ions or electrons.
  • Preferably polymerization is carried out by UV irradiation.
  • a source for actinic radiation for example a single UV lamp or a set of UV lamps can be used.
  • Another possible source for actinic radiation is a laser, like e.g. a UV laser, an IR laser or a visible laser.
  • the polymerization is carried out in the presence of an initiator absorbing at the wavelength of the actinic radiation.
  • an initiator absorbing at the wavelength of the actinic radiation.
  • a photoinitiator can be used that decomposes under UV irradiation to produce free radicals or ions that start the polymerization reaction.
  • a radical photoinitiator is used
  • curing polymerizable mesogens vinyl and epoxide groups preferably a cationic photoinitiator is used.
  • a photoinitiator for radical polymerization for example the commercially available Irgacure 651, Irgacure 184, Darocure 1173 or Darocure 4205 (all from Ciba Geigy AG) can be used, whereas in case of cationic photopolymerization the commercially available UVI 6974 (Union Carbide) can be used.
  • the polymerizable mesogenic material preferably comprises 0.01 to 10 %, very preferably 0.05 to 5 %, in particular 0.1 to 3 % of a polymerization initiator.
  • UV photoinitiators are preferred, in particular radicalic UV photoinitiators.
  • a second substrate that does not only aid alignment of the polymerizable composition, but also excludes oxygen that may inhibit the polymerization.
  • the curing can be carried out under an atmosphere of inert gas.
  • curing in air is also possible using suitable photoinitiators and high lamp power.
  • oxygen exclusion most often is not needed, but water should be excluded.
  • the polymerization of the polymerizable mesogenic material is carried out under an atmosphere of inert gas, preferably under a nitrogen atmosphere.
  • the polymerizable material may also comprise one or more other suitable components such as, for example, catalysts, stabilizers, chain-transfer agents, co-reacting monomers or surface-active compounds.
  • suitable components such as, for example, catalysts, stabilizers, chain-transfer agents, co-reacting monomers or surface-active compounds.
  • stabilizers is preferred in order to prevent undesired spontaneous polymerization of the polymerizable material for example during storage.
  • planar alignment i.e. an orientation wherein the mesogens are aligned substantially parallel to the plane of the layer, with a low tilt angle relative to the plane of the layer.
  • Suitable surfactants are described for example in J. Cognard, Mol.Cryst.Liq.Cryst. 78, Supplement 1, 1-77 (1981) .
  • Particularly preferred are non-ionic surfactants, such as mixtures of non-ionic fluoroalkylalkoxylate surfactants selected from formula III and IV C n F 2n+1 SO 2 N(C 2 H 5 )(CH 2 CH 2 O) x CH 3 III C n F 2n+1 (CH 2 CH 2 O) x H IV wherein n is an integer from 4 to 12 and x is an integer from 5 to 15.
  • non-ionic surfactants such as mixtures of non-ionic fluoroalkylalkoxylate surfactants selected from formula III and IV C n F 2n+1 SO 2 N(C 2 H 5 )(CH 2 CH 2 O) x CH 3 III C n F 2n+1 (CH 2 CH 2 O) x H IV wherein n is an integer from 4 to 12 and x is
  • the surfactants of formula III are commercially available under the trade name Fluorad 171 (from 3M Co.), the surfactants of formula IV under the trade name Zonyl FSN (from DuPont).
  • Polymerizable mesogenic mixtures according to this preferred embodiment contain preferably from 500 to 2500 ppm, in particular from 1000 to 2500 ppm, very preferably from 1500 to 2500 ppm of surfactants.
  • Typical examples for difunctional non mesogenic monomers are alkyldiacrylates or alkyldimethacrylates with alkyl groups of 1 to 20 C atoms.
  • Typical examples for non mesogenic monomers with more than two polymerizable groups are trimethylpropanetrimethacrylate or pentaerythritoltetraacrylate.
  • the polymerizable mixture comprises up to 70%, preferably 3 to 50 % of a non mesogenic compound with one polymerizable functional group.
  • Typical examples for monofunctional non mesogenic monomers are alkylacrylates or alkylmethacrylates.
  • the polymerization has to be carried out in the liquid crystal phase of the polymerizable mesogenic mixture. Therefore, preferably polymerizable mesogenic compounds or mixtures with low melting points and broad liquid crystal phase ranges are used.
  • the use of such materials allows to reduce the polymerization temperature, which makes the polymerization process easier and is a considerable advantage especially for mass production.
  • suitable polymerization temperatures depends mainly on the clearing point of the polymerizable material and inter alia on the softening point of the substrate.
  • the polymerization temperature is at least 30 degrees below the clearing temperature of the polymerizable mesogenic mixture.
  • Polymerization temperatures below 120 °C are preferred. Especially preferred are temperatures below 90 °C, in particular temperatures of 60 °C or less.
  • the retardation value of optical retardation films like those obtained by the inventive process is depending inter alia on the type of liquid crystal materials used and on the thickness of the polymer film.
  • a liquid crystal material with a birefringence of about 0.15 an increase in optical retardation of 100 nm requires an increase in layer thickness of about 1 ⁇ m.
  • the retardation of the inventive optical retardation films is preferably ranging from about 20 to about 700 nm.
  • the layer thickness of the optical retardation film is preferably from 1 to 30 ⁇ m, in particular from 1 to 15 ⁇ m, very preferably from 1 to 7 ⁇ m.
  • An optical retardation film prepared from the inventive process can be used in liquid crystal displays as quarter wave retardation film, like for example described in the cited prior art document WO 98/04651- A, or as a compensation film, like for example disclosed in the WO 98/12584-A .
  • an optical retardation film prepared by the process of the invention is used in combination together with a broadband reflective polarizer as disclosed in the WO 97/35219 and/or a conventional linear polarizer.
  • An optical retardation film which is obtained by the inventive process exhibits at least one layer of an anisotropic polymer with a major optical axis substantially parallel to the plane of the layer.
  • Films thus obtained and having an optical retardation from 90 to 180 nm are typically used as a quarter wave retardation film as suggested e.g. in the WO 98/04651-A , whereas films thus obtained with an optical retardation from 400 to 700 nm are typically used as a compensation film e.g. for STN displays.
  • Another optical retardation film which is obtained by the inventive process exhibits at least one layer of an anisotropic polymer with a major optical axis that is tilted at an angle ⁇ o of between 0 and 90 degrees relative to the plane of the layer.
  • films with a tilt angle that is substantially zero and a value of the optical retardation as low as 20 nm are used.
  • an optical film exhibiting a splayed structure i.e. wherein the major optical axis exhibits a tilt angle ⁇ o between 0 and 90 degrees that varies in a direction normal to the film plane, and having a retardation from 40 to 100 nm at normal incidence is preferred.
  • Yet another optical retardation film which is obtained by the inventive process exhibits at least one layer of an anisotropic polymer, wherein the major optical axis is either parallel or tilted at an angle ⁇ o of between 0 and 90 degrees relative to the plane of the layer, and the optical axis is additionally being inclined at an angle ⁇ LC of between 0 and 90 degrees relative to the long side of the film.
  • Such an optical retardation film can be obtained on a substrate that has been rubbed according to the inventive method, with the rub angle ⁇ being different from 0 degrees, and the angle of inclination ⁇ LC of the optical axis being approximately identical to the effective rub angle ⁇ eff . It can be used as a compensation film as suggested e.g. in the WO 98/12584-A .
  • the compounds (1) to (3) can be prepared according to or in analogy to the methods described in D.J.Broer et al., Makromol.Chem. 190, 3201-3215 (1989 ) and in the WO 97/34862 .
  • Irgacure 907 is a radicalic photoinitiator (available from Ciba Geigy).
  • FC 171 is a non-ionic fluorocarbon surfactant (from 3M Corp.).
  • the mixture was dissolved in a concentration of 21 % by weight in toluene.
  • the solution was filtered to remove impurities and small particles
  • the solution of the polymerizable mesogenic mixture from example 1 was coated by means of a doctor blade as a thin layer with a thickness of approximately 12 ⁇ m onto the individual samples of rubbed TAC web.
  • the solvent was allowed to evaporate at 55 °C
  • the remaining approximately 1-2 ⁇ m thick layer of the polymerizable liquid crystalline mixture was then cured in its nematic phase in air at ambient temperature by irradiating with UV light from a mercury lamp for a few seconds.
  • the angle of inclination ⁇ LC of the mesogenic groups in the individual samples of liquid crystal polymer film relative to the film plane was measured using a protractor and a ruler, viewing the film between crossed polarizers.
  • the angle ⁇ LC was measured relative to the straight edge of the substrate which is corresponding to the long side of the long TAC web in the rubbing process.
  • the measured angle ⁇ LC was compared with the calculated effective rub angle ⁇ eff of the web, as shown in Table 1A above.
  • the solution of the polymerizable mesogenic mixture from example 1 was coated onto the individual samples of the thus rubbed TAC web and the mixture was cured as described in use example 1A.
  • the inclination angle ⁇ LC of the mesogenic groups was measured and compared with the calculated effective rub angle ⁇ eff of the TAC web, as shown in Table 1B above.
  • the solution of the polymerizable mesogenic mixture from example 1 was coated onto the individual samples of the thus rubbed TAC web and the mixture was cured as described in use example 1A.
  • the inclination angle ⁇ LC of the mesogenic groups was measured and compared with the calculated effective rub angle ⁇ eff of the TAC web, as shown in Table 1C above.

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Claims (10)

  1. Verfahren zur Herstellung einer optischen Verzögerungsfolie, enthaltend mindestens eine Schicht eines anisotropen Polymermaterials, wobei das Verfahren die folgenden Schritte umfasst:
    A) Reiben eines sich fortlaufend bewegenden Substrates (10) mit mindestens einer Reibungswalze (11;12),
    B) Aufbringen einer Schicht eines polymerisierbaren mesogenen Materials, eines Flüssigkristallpolymers oder eines Flüssigkristalloligomers auf die geriebene Oberfläche des Substrates (10),
    C) gegebenenfalls Polymerisieren und/oder Vernetzen des beschichteten Materials von Schritt B) durch Einwirkung von Wärme oder aktinischer Strahlung,
    D) gegebenenfalls Wiederholen der Schritte B) und C) mindestens noch einmal, und
    E) gegebenenfalls Entfernen des Polymerfilms von dem Substrat (10),
    wobei das Reiben des Substrats (10) in Schritt A) durch kontinuierliches Bewegen des Substrates (10) in der Richtung (13;13*) zwischen mindestens zwei Walzen (11;12) erfolgt, von denen mindestens eine Walze eine Reibungsbehandlung auf das Substrat (10) ausübt, und mindestens eine der mindestens zwei Walzen (11;12) derart angeordnet ist, dass das Substrat (10) zumindest partiell um die Walze (11;12) in einem Wicklungswinkel φ gewickelt ist, der im Bereich von 0 bis mindestens 270 Grad variiert werden kann, wobei der Wicklungswinkel φ definiert ist als der Winkel des kreisförmigen Segmentes des Querschnittes der Walze (11;12), die mit dem sich bewegenden Substrat (10) in Kontakt ist, dadurch gekennzeichnet, dass
    a) sich die mindestens zwei Walzen (11;12) auf gegenüberliegenden Seiten des Substrates (10) befinden,
    b) sich die mindestens zwei Walzen (11;12) gegen die Bewegungsrichtung (13;13*) des Substrates (10) drehen;
    c) die mindestens zwei Walzen (11;12) derart angeordnet sind, dass ihre Rotationsachsen (14,15) und die Normale zur Bewegungsrichtung (13) des Substrates (10) in der Ebene des Substrates (10) einen Winkel θ (Reibungswinkel) bilden, der im Bereich von 0 bis weniger als 90 Grad variiert werden kann.
  2. Verfahren nach Anspruch 1, dadurch gekennzeichnet, dass der Wicklungswinkel von mehr als 0 bis zu 180 Grad reicht.
  3. Verfahren nach mindestens einem der Ansprüche 1 und 2, dadurch gekennzeichnet, dass der Reibungswinkel von mehr als 0 bis zu 60 Grad reicht.
  4. Verfahren nach mindestens einem der Ansprüche 1 bis 3, dadurch gekennzeichnet, dass mindestens eine der mindestens zwei Walzen (11;12), die eine Reibungsbehandlung auf Substrat (10) ausüben, mit einem Reibungstuch bedeckt ist.
  5. Verfahren nach mindestens einem der Ansprüche 1 bis 4, dadurch gekennzeichnet, dass in Schritt B) das Substrat (10) mit einer Schicht eines poylmerisierbaren mesogenen Materials beschichtet ist, das mindestens eine polymerisierbare mesogene Verbindung mit mindestens einer polymerisierbaren Gruppe enthält.
  6. Vorrichtung zum Reiben eines Substrates (10), die mindestens zwei Walzen (11;12) enthält, von denen mindestens eine dazu ausgelegt ist, dass sie eine Reibungsbehandlung auf das Substrat (10) ausübt, Einrichtungen zum kontinuierlichen Bewegen des Substrates (10) in einer Richtung (13;13*) zwischen mindestens zwei Walzen (11;12) und die gegebenenfalls eine oder mehrere Führungswalzen (16;17) enthält, wobei mindestens eine der mindestens zwei Walzen (11; 12) derart angeordnet ist, dass das Substrat (10) zumindest partiell um die Walze (11; 12) in einem Wicklungswinkel φ gewickelt ist, der im Bereich von 0 bis mindestens 270 Grad variiert werden kann, wobei der Wicklungswinkel φ definiert ist als der Winkel des kreisförmigen Segmentes des Querschnittes der Walze (11 ;12), die mit dem sich bewegenden Substrat (10) in Kontakt ist, dadurch gekennzeichnet, dass
    a) sich die mindestens zwei Walzen (11;12) auf gegenüberliegenden Seiten des Substrates (10) befinden,
    b) sich die mindestens zwei Walzen (11;12) gegen die Bewegungsrichtung (13;13*) des Substrates (10) drehen;
    c) die mindestens zwei Walzen (11;12) derart angeordnet sind, dass ihre Rotationsachsen (14,15) und die Normale zur Bewegungsrichtung (13) des Substrates (10) in der Ebene des Substrates (10) einen Winkel θ (Reibungswinkel) bilden, der im Bereich von 0 bis weniger als 90 Grad variiert werden kann.
  7. Vorrichtung nach Anspruch 6, dadurch gekennzeichnet, dass die Rotationsachsen (14;15) der mindestens zwei Walzen (11;12) über mindestens ein Verbindungselement (22) verbunden sind, wobei die Walzen (11; 12) um eine Achse (23) parallel zu ihren Rotationsachsen (14;15) gedreht werden können.
  8. Vorrichtung nach Anspruch 6 oder 7, dadurch gekennzeichnet, dass mindestens zwei Walzen (11;12) eine drehbare Einheit (25) bilden, die weiterhin eine Antriebsvorrichtung (26) mit einem oder mehreren Motoren und gegebenenfalls Getrieben enthält, die den Walzen (11;12) Rotationsgeschwindigkeit mit gleichem Absolutwert und im Gegensinn verleihen, wobei die Einheit (25) drehbar ist um eine Achse (27), die senkrecht zu der Ebene ist, die von den Führungswalzen (16;17) umgrenzt wird, und um eine Achse (28), die parallel zu den Rotationsachsen (14;15) der Walzen (11;12) ist.
  9. Vorrichtung nach mindestens einem der Ansprüche 6 bis 8, dadurch gekennzeichnet, dass sie weiterhin ein oder mehrere Vakuumbetten (30;31) enthält, über die das Substrat (10) vor und/oder nach der Reibung bewegt wird.
  10. Vorrichtung nach mindestens einem der Ansprüche 6 bis 9, dadurch gekennzeichnet, dass sie mindestens drei Walzen (11;12;32) enthält, die eine Reibungsbehandlung auf das Substrat (10) ausüben, von denen sich mindestens zwei auf gegenüberliegenden Seiten des Substrates (10) befinden.
EP99114401A 1998-07-31 1999-07-22 Verfahren zur Herstellung eines optischen Verzögerungsfilms Expired - Lifetime EP0977075B1 (de)

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